Knowledge
Metrology Dictionary
The complete VIM vocabulary — all numbered concepts of JCGM 200:2012 as faithful paraphrases with clause citations — plus clearly labeled practice terms from the GUM, ILAC, ISO and NIST guidance.
Combined standard measurement uncertainty
The standard uncertainty obtained from the individual standard uncertainties (and covariances) of the input quantities through the measurement model — root-sum-of-squares in the uncorrelated case, denoted u꜀.
VIM (JCGM 200:2012) 2.31 · Uncertainty
Coverage factor
The number k by which a combined standard uncertainty is multiplied to obtain an expanded uncertainty; k = 2 is standard certificate practice, with a t-based k when effective degrees of freedom are small.
VIM (JCGM 200:2012) 2.38 · Uncertainty
Coverage interval
The interval containing the set of true quantity values of a measurand with a stated probability, based on available information — unlike a classical confidence interval, not necessarily centred on the measured value.
VIM (JCGM 200:2012) 2.36 · Uncertainty
Coverage probability
The probability that the set of true quantity values of a measurand is contained within a specified coverage interval — commonly 95 % in calibration practice.
VIM (JCGM 200:2012) 2.37 · Uncertainty
Definitional uncertainty
The component of uncertainty resulting from the finite detail in the definition of the measurand itself — the floor below which no better procedure can go without redefining the quantity.
VIM (JCGM 200:2012) 2.27 · Uncertainty
Expanded measurement uncertainty
The product of a combined standard uncertainty and a coverage factor greater than one: U = k·u꜀. With k = 2 the coverage probability is about 95 % for a normal distribution.
VIM (JCGM 200:2012) 2.35 · Uncertainty
Input quantity in a measurement model
A quantity that must be measured, or whose value can be otherwise obtained, to calculate a measured value of the measurand.
VIM (JCGM 200:2012) 2.50 · Uncertainty
Measurement function
The function whose output value, computed from the input quantity values, is the measured value of the output quantity — Y = f(X₁, …, Xₙ) when the model can be solved explicitly.
VIM (JCGM 200:2012) 2.49 · Uncertainty
Measurement model
The mathematical relation among all quantities known to be involved in a measurement, of the general form f(Y, X₁, …, Xₙ) = 0.
VIM (JCGM 200:2012) 2.48 · Uncertainty
Measurement uncertainty
A non-negative parameter characterizing the dispersion of the quantity values being attributed to a measurand, based on the information used — the quantitative expression of doubt about a result.
VIM (JCGM 200:2012) 2.26 · Uncertainty
Monte Carlo method (uncertainty)
Propagation of probability distributions through a measurement model by repeated random sampling, per JCGM 101:2008. Preferred when the model is strongly non-linear or distributions are non-normal.
JCGM 101:2008 · Uncertainty
Output quantity in a measurement model
The quantity whose measured value is calculated from the input quantity values using the model — the measurand's seat in the mathematics.
VIM (JCGM 200:2012) 2.51 · Uncertainty
Rectangular distribution
A probability distribution assigning equal likelihood everywhere between limits ±a. Used for Type B components where only limits are known; its standard uncertainty is a/√3.
GUM (JCGM 100:2008) 4.3.7 · Uncertainty
Relative standard measurement uncertainty
A standard uncertainty divided by the absolute value of the measured quantity value — the form specifications and CMCs often quote (parts in 10⁶, percent of reading).
VIM (JCGM 200:2012) 2.32 · Uncertainty
Sensitivity coefficient
The partial derivative ∂f/∂xᵢ of the measurement model with respect to an input quantity, evaluated at the estimates: it converts an input standard uncertainty into its contribution to the uncertainty of the result.
GUM (JCGM 100:2008) 5.1.3 · Uncertainty
Standard measurement uncertainty
Measurement uncertainty expressed as a standard deviation, denoted u — the common currency into which all components are converted before combination.
VIM (JCGM 200:2012) 2.30 · Uncertainty
Target measurement uncertainty
An uncertainty specified in advance as an upper limit, decided from the intended use of the result.
VIM (JCGM 200:2012) 2.34 · Uncertainty
Triangular distribution
A probability distribution rising linearly to a peak midway between limits ±a. Used when values near the centre are more likely; its standard uncertainty is a/√6.
GUM (JCGM 100:2008) 4.3.9 · Uncertainty
Type A evaluation of measurement uncertainty
Evaluation of a component of uncertainty by statistical analysis of measured values obtained under defined conditions — e.g. the standard deviation of the mean of repeated readings.
VIM (JCGM 200:2012) 2.28 · Uncertainty
Type B evaluation of measurement uncertainty
Evaluation of a component of uncertainty by means other than statistical analysis of current measurements — calibration certificates, specifications, handbook data, assumed distributions.
VIM (JCGM 200:2012) 2.29 · Uncertainty
Uncertainty budget
A statement of a measurement uncertainty, its components, and their calculation and combination — usually tabulated with each source, evaluation type, distribution, standard uncertainty, sensitivity coefficient, and contribution.
VIM (JCGM 200:2012) 2.33 · Uncertainty
Welch–Satterthwaite formula
The formula for estimating the effective degrees of freedom of a combined standard uncertainty from the degrees of freedom of its components — used to select a coverage factor from the t-distribution.
GUM (JCGM 100:2008) G.4 · Uncertainty